Analog-to-digital converters are used in combination with digital systems for a host of different industrial and consumer applications. However, these different applications require different analog-to-digital converter configurations, e.g., number of sample channels and bit resolutions. For example, a motor control device may use a 4-channel 10-bit analog-to-digital converter (ADC), while a general purpose application device may employ a single channel 12-bit ADC.
Generally, a successive approximation register ADC (SAR ADC) may be used for 10 and 12 bit resolution conversions. The SAR ADC is low in cost, consumes low operating power, and has reasonable conversion speeds. Also a sample and hold circuit may be used in combination with the SAR ADC. The sample and hold circuit comprises capacitors and switches to couple the capacitors to an analog voltage to be converted. This analog voltage is sampled and held in the capacitors. Then the voltage sample held in the capacitors is coupled to the ADC comparator for conversion of the analog voltage to a digital representation (e.g., 10 or 12 bits).
The capacitors in the SAR ADC may serve two functions: sampling and conversion. During a sampling phase the capacitors may store the analog voltage to be converted, e.g., a voltage difference between the analog voltage and an internal reference voltage, for example Vdd/2. The capacitance value of the capacitors is not significant during the sampling phase. The sampling phase ends and hold phase begins when the switches between the capacitors and the internal reference are opened. The next phase is conversion. During the conversion phase, ends of the capacitors coupled to a voltage comparator are allowed to float, e.g., not driven. Then during a successive approximation process, the other ends of the capacitors are switched in binary weighted groups to a voltage reference, e.g., Vrefh or Vrefl (e.g., ground or common voltage) by a successive approximation register (SAR) and control circuitry. During this conversion process, the capacitance values of the capacitors are very significant. The quality of capacitance value matching of the capacitors may determine the differential non-linearity (DNL) and integral non-linearity (INL) performance of the SAR ADC.